The SIP REFER method relies on the SIP-Specific Event Notification framework. That framework was revised by RFC 6665. This document highlights the implications of the requirement changes in RFC 6665@ and updates the definition of the REFER method described in RFC 3515 to clarify and disambiguate the impact of those changes.
RFC 7647-2015 history
2015RFC 7647-2015 Clarifications for the Use of REFER with RFC 6665